Solenoid with variable reluctance plunger

ABSTRACT

A solenoid for a vehicle starter includes at least one coil with a passage extending through the coil in an axial direction. The solenoid further includes a plunger configured to move in the axial direction within the passage. The plunger includes a cylindrical outer surface with a substantially uniform diameter and a circumferential notch. The cylindrical outer surface includes a first portion with a first diameter on one side of the circumferential notch, and a second portion with the first diameter on an opposite side of the circumferential notch. The circumferential notch includes a portion with a second diameter that is less than the first diameter.

FIELD

This relates to the field of vehicle starters, and more particularly, tosolenoids for starter motor assemblies.

BACKGROUND

Starter motor assemblies that assist in starting engines, such asengines in vehicles, are well known. A conventional starter motorassembly is shown in FIG. 15. The starter motor assembly 200 of FIG. 23includes a solenoid 210, an electric motor 202, and a drive mechanism204. The solenoid 210 includes a coil 212 that is energized by a batteryupon the closing of an ignition switch. When the solenoid coil 212 isenergized, a plunger 216 moves in a linear direction, causing a shiftlever 205 to pivot, and forcing a pinion gear 206 into engagement with aring gear of a vehicle engine (not shown). When the plunger 216 reachesa plunger stop, electrical contacts are closed connecting the electricmotor 202 to the battery. The energized electric motor 202 then rotatesand provides an output torque to the drive mechanism 204. The drivemechanism 204 transmits the torque of the electric motor through variousdrive components to the pinion gear 206 which is engaged with the ringgear of the vehicle engine. Accordingly, rotation of the electric motor202 and pinion 206 results in cranking of the engine until the enginestarts.

Many starter motor assemblies, such as the starter motor assembly 200 ofFIG. 15 are configured with a “soft-start” starter motor engagementsystem. The intent of a soft start starter motor engagement system is tomesh the pinion gear of the starter into the engine ring gear beforefull electrical power is applied to the starter motor. If the pinionring gear abuts into the ring gear during this engagement, the motorprovides a small torque to turn the pinion gear and allow it to properlymesh into the ring gear before high current is applied. Theconfiguration of the solenoid, shift yoke, electrical contacts, andmotor drive are such that high current is not applied to the motorbefore the gears are properly meshed. Accordingly, milling of the piniongear and the ring gear is prevented in a starter motor with a soft-startengagement system.

Starters with a soft start engagement system, such as that of FIG. 15,typically include a solenoid with two distinct coils. The first coil isa pull-in coil 212 and the second coil is a hold in coil 214. As shownin FIG. 15, the pull-in coil 212 is wound first on the spool 220. On topof this winding the hold-in coil 214 is wound. Sometimes this order isreversed such that the hold-in coil 214 is wound first on the spool 220followed by the pull-in coil 212.

During operation of the starter, the closing of the ignition switch(typically upon the operator turning a key) energizes both the pull-incoil 212 and the hold-in coil 214. Current flowing through the pull-incoil 212 at this time also reaches the electric motor 202, applying somelimited power to the electric motor, and resulting in some low torqueturning of the pinion. Energization of the pull-in coil 212 and hold-incoil 214 moves a solenoid shaft (also referred to herein as the“plunger”) in an axial direction. The axial movement of the solenoidplunger moves the shift lever 205 and biases the pinion gear 206 towardengagement with the engine ring gear. Once the solenoid plunger reachesthe plunger stop, a set of electrical contacts is closed, therebydelivering full power to the electrical motor. Closing of the electricalcontacts effectively short circuits the pull-in coil 212, eliminatingunwanted heat generated by the pull-in coil. However, with the pull-incoil is shorted, the hold-in coil 214 provides sufficientelectromagnetic force to hold the plunger in place and maintain theelectrical contacts in a closed position, thus allowing the delivery offull power to continue to the electric motor 202. The fully poweredelectric motor 202 drives the pinion gear 206, resulting in rotation ofthe engine ring gear, and thereby cranking the vehicle engine.

After the engine fires (i.e., vehicle start), the operator of thevehicle opens the ignition switch. The electrical circuit of the startermotor assembly is configured such that opening of the ignition switchcauses current to flow through the hold-in coil and the pull-in coil inopposite directions. The pull-in coil 212 and the hold-in coil 214 areconfigured such that the electromagnetic forces of the two coils 212,214 cancel each other upon opening of the ignition switch, and a returnspring forces the plunger 216 back to its original un-energizedposition. As a result, the electrical contacts that connected theelectric motor 202 to the source of electrical power are opened, and theelectric motor is de-energized.

In order to produce a high performing vehicle starter with a soft startmotor engagement system, such as that described above, designers arefaced with numerous design challenges. First, the pull-in coil must beproperly designed to avoid various issues that may arise duringoperation of the starter. As described above, when the pull-in coil of asoft-start starter motor engagement system is energized (i.e., when theignition switch contacts close due to operator turning engine switch keyon), the pull-in coil provides electromagnetic force to pull the plungertoward the plunger stop and to the closed position. However, the pull-incoil is connected electrically in series with the starter motor, andshould only have a low resistance. With low resistance through thepull-in coil, sufficient current flows through the pull-in coil and tothe electric motor such that the electric motor can deliver a sufficientoutput torque to rotate the pinion gear and avoid abutment with the ringgear, as described previously. This required torque is typically 8-12N-m. For a 12V motor, the resistance may be on the order of 0.030 ohmsso that several hundred amps flow through the motor, and also the seriesconnected pull-in coil, during soft start. However, this low ofresistance of the pull-in coil creates other design challenges. First,if the soft start period is prolonged, or repetitive starts areperformed, a high amount of ohmic heat is generated in the pull-in coilbecause of the large amount of current flowing through the pull-in coil.For a 12V system this can be on the order of 3-4 kW, and this can leadto thermal failure of the insulation system of the wiring that forms thecoils. Second, the large current through the pull-in coil creates a muchstronger electromagnetic force on the plunger during closure than isneeded. This may become a problem when an abutment between the piniongear and ring gear occurs, and the impact force of the pinion gear onthe ring gear can exceed 4500N. As a result, the ring gear couldfracture or chip. Over time and thousands of starts, the surface of thering gear may deteriorate and require replacement for proper starting.

Design challenges related to the pull-in coil, such as those discussedin the preceding paragraph result in additional design challenges withrespect to other components of the starter, such as the hold-in coil.For example, as discussed in the previous paragraph, the pull-in coilhas specific design limitations related to the current flowing throughthe pull-in coil. Since the electromagnetic excitation is the product ofcoil turns times current, and since current is fixed, this generallyleaves the number of turns of the pull-in coil as the primary designvariable for the pull-in coil. While the number of turns of the pull-incoil can be reduced to reduce the impact abutment force issue describedpreviously, this presents a problem with the hold-in coil. Inparticular, the number of turns in the hold-in coil should match thepull-in coil so that during disengagement of the pinion gear and thering gear following vehicle start, the electromagnetic forces of the twocoils will cancel each other and allow the pinion gear to pull cleanlyout of the ring gear. However, before vehicle start, the hold-in coilstays energized for a much longer period of time than the pull-in coil.Therefore, the hold-in coil should not be of low resistance or it willthermally fail. Thus, the resistance of the hold-in coil generally is anorder of magnitude higher than that of the pull-in coil. The highresistance of the hold-in coil means that current flow through thehold-coil before start is relatively low, resulting in a relatively lowamp-turn product. If the number of turns of the hold-in coil is too low,then the hold-in coil will deliver an insufficient magnetic force tohold the plunger closed and the starter motor will disengage beforevehicle start.

As explained in the previous paragraphs, designers of vehicle starterswith soft start motor engagement systems are faced with opposing designchallenges for two coils that should produce equivalent electromagneticforces. On the one hand designers strive to limit the turns of thepull-in coil in order to reduce the impact force during engagement ofthe pinion gear and the ring gear. On the other hand designers strive toincrease the turns of the hold-in coil such that the hold-in coildelivers sufficient electromagnetic force to maintain the plunger in aclosed position during engine cranking. Accordingly, it would bedesirable to provide a solenoid for a vehicle starter with a pull-incoil that limits the impact force during engagement of the pinion gearand the ring gear. It would also be desirable to provide a hold-in coilfor the solenoid that delivers sufficient electromagnetic force tomaintain the plunger in a closed position during engine cranking.Additionally, it would be desirable if such a solenoid were relativelysimple in design and inexpensive to implement.

SUMMARY

In accordance with one exemplary embodiment of the disclosure, there isprovided a solenoid for a vehicle starter. The solenoid includes atleast one coil with a passage extending through the coil in an axialdirection. The solenoid further includes a plunger configured to move inthe axial direction within the passage. The plunger includes acylindrical outer surface with a substantially uniform diameter and acircumferential notch. The cylindrical outer surface includes a firstportion with a first diameter on one side of the circumferential notch,and a second portion with the first diameter on an opposite side of thecircumferential notch. The circumferential notch includes a portion witha second diameter that is less than the first diameter.

In at least one embodiment a solenoid for a vehicle starter comprises atleast one coil with a passage extending through the coil. The solenoidfurther comprises a substantially solid plunger positioned within thepassage and configured to slide within the passage in an axial directionbetween a first position and a second position. The plunger includes acylindrical outer surface with a circumferential notch formed in thecylindrical outer surface between a first end and a second end of thecylindrical outer surface. A radial wall is separated from the plungerby a radial distance, and the radial distance varies when the plungermoves from the first position to the second position.

In at least one embodiment, a method of operating a solenoid for avehicle starter comprises energizing at least one coil of the solenoid.The method further comprises moving a substantially solid plunger in anaxial direction as a result of energization of the at least one coil,the substantially solid plunger including a cylindrical outer surfacewith a substantially uniform diameter and a circumferential notch formedin the cylindrical outer surface between a first end and a second end ofthe cylindrical outer surface. The method also includes varying themagnetic reluctance between the plunger and a solenoid wall as a resultof the circumferential notch moving relative to the solenoid wall.

The above described features and advantages, as well as others, willbecome more readily apparent to those of ordinary skill in the art byreference to the following detailed description and accompanyingdrawings. While it would be desirable to provide a solenoid thatprovides one or more of these or other advantageous features, theteachings disclosed herein extend to those embodiments which fall withinthe scope of the appended claims, regardless of whether they accomplishone or more of the above-mentioned advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a vehicle starter including a motorand solenoid;

FIG. 2 shows a perspective view of a spool, pull-in coil, and hold-incoil of the solenoid of FIG. 1;

FIG. 3 shows a diagram illustrating lines of magnetic flux through thesolenoid when the pull-in coil and hold-in coil of FIG. 2 are energizedand the plunger is removed from a plunger stop;

FIG. 4 shows a diagram illustrating lines of magnetic flux through thesolenoid when the pull-in coil and hold-in coil of FIG. 2 are energizedand the plunger is in transition toward the plunger stop;

FIG. 5 shows a diagram illustrating lines of magnetic flux through thesolenoid when only the hold-in coil of FIG. 2 is energized and theplunger is engaged with the plunger stop;

FIG. 6 shows a cross-sectional view of the spool of FIG. 2 taken along acenterline of the spool;

FIG. 6A shows a cross-sectional view of the spool along line A-A of FIG.6, illustrating one side of a middle flange of the spool;

FIG. 6B shows a cross-sectional view of the spool along line B-B of FIG.6, illustrating another side of the middle flange of the spool;

FIG. 6C shows an side view of the spool along line C-C of FIG. 6,illustrating an end flange of the spool;

FIG. 7 shows a perspective view of an alternative embodiment of thespool of FIG. 2;

FIG. 8 shows the spool of FIG. 7 with the hold-in coil being wound inone direction on a second coil bay of the spool;

FIG. 9 shows the spool of FIG. 8 with the hold-in coil being wound in anopposite direction on the second coil bay of the spool;

FIG. 10 shows the spool of FIG. 9 with the hold-in coil completely woundon the second coil bay of the spool;

FIG. 11 shows the spool of FIG. 10 with the pull-in coil being wound ona first coil bay of the spool;

FIG. 12 shows the spool of FIG. 11 with the pull-in coil completelywound on the first coil bay of the spool;

FIG. 13 shows a cross-sectional view of the spool along line D-D of FIG.12, including the hold-in coil and pull-in coil positioned on the spool;

FIG. 14 shows a cross-sectional view of an alternative embodiment of thespool, hold-in coil and pull-in coil of FIG. 13;

FIG. 15 shows a cross-sectional view of the spool, pull-in coil, andhold-in coil of FIG. 2 with an alternative embodiment of a solenoidplunger with circumferential notch positioned within the interiorpassage;

FIG. 16 shows a graph illustrating the difference in plunger axial forcebetween a standard plunger and the variable reluctance plunger of FIG.15 as the axial plunger gap is closed;

FIG. 17 shows a cross-sectional view of the position of thecircumferential notch when the plunger is in position A of FIG. 16;

FIG. 18 shows a cross-sectional view of the position of thecircumferential notch when the plunger is in position B of FIG. 16;

FIG. 19 shows a cross-sectional view of the position of thecircumferential notch when the plunger is in position C of FIG. 16;

FIG. 20 shows a cross-sectional view of the position of thecircumferential notch when the plunger is in position D of FIG. 16;

FIG. 21 shows an isolated side view of the plunger with circumferentialnotch of FIG. 15 with a sleeve member positioned over thecircumferential notch;

FIG. 21A shows a perspective view of one embodiment of the sleeve memberof FIG. 21A;

FIG. 21B shows a perspective view of another embodiment of the sleevemember of FIG. 21A;

FIG. 22 shows a cross-sectional view of an alternative embodiment of theplunger, spool, pull-in coil, and hold-in coil of FIG. 15 with thesleeve of FIG. 21 positioned on the plunger; and

FIG. 23 shows a cutaway view of a conventional starter motor with a softstart starter motor engagement system

DESCRIPTION

General Starter Arrangement

With reference to FIG. 1, in at least one embodiment a starter 100 for avehicle comprises an electric motor 102 and a solenoid 110. Although notshown in the FIG. 1, the starter 100 also includes a drive mechanism andpinion gear, similar to the conventional starter assembly 200 describedabove with reference to FIG. 15. The electric motor 102 in theembodiment of FIG. 1 is positioned in a motor circuit 104 that isconfigured to connect the motor to the vehicle battery (not shown) viathe B+ terminal. The solenoid 110 is positioned in the motor circuit 104to facilitate connection of the motor to the vehicle battery. Thesolenoid includes a pull-in coil 112, a hold-in coil 114, a plunger 116,and an ignition switch 118.

The motor circuit 104 of FIG. 1 includes a first current path 106 and asecond current path 108 configured to provide electrical power to theelectric motor 102. The first current path 106 begins at the B+terminal, travels across the contacts 119 of the ignition switch 118,continues to node 115, travels through the pull-in coil, and ends at theinput terminal 103 of the electric motor 102. Accordingly, this firstcurrent path 106 is only a closed path when the contacts 119 of theignition switch 118 are closed.

The second current path 108 begins at the B+ terminal, travels acrossthe motor contacts 117 associated with the plunger 116 and ends at theinput terminal 103 of the electric motor 102. Accordingly, this secondcurrent path 108 is only a closed path when the plunger 116 has closedthe motor contacts 117. Moreover, when the second current path 108 isclosed, the first current path 106 is shorted by the second current path108, and no current flows through the pull-in coil 112. Upon closing ofthe ignition switch 118, the solenoid 110 and motor 102 cooperate toprovide a soft start motor engagement system for a vehicle.

Axially Adjacent Coils

FIG. 2 shows the pull-in coil 112 and the hold-in coil 114 of thesolenoid 110 positioned on a spool 120 of the solenoid 110. In theembodiment of FIG. 2, the pull-in coil 112 and the hold-in coil 114 areadjacent to one another in an axial direction of the spool 120. Theaxial direction is represented in FIG. 2 by axis 132.

The pull-in coil 112 is comprised of a first length of wire wound arounda first portion of the spool 120 to form a first plurality of conductorwindings (i.e., turns). The wire for the pull-in coil 112 has arelatively large cross-sectional area such that the resistance of theconductor windings is relatively low. Similarly, the hold-in coil 114 iscomprised of a second length of wire wound around a second portion ofthe spool to form a second plurality of conductor windings (i.e.,turns). The wire for the hold-in coil 114 is has a relatively smallcross-sectional area such that the resistance of the conductor windingsis relatively high.

The pull-in coil 112 and the hold-in coil 114 are retained in aside-by-side arrangement on the spool 120. In the embodiment of FIG. 2,the spool 120 is a single component comprised of a glass-filled nylonmaterial. However, it will be recognized that the spool mayalternatively be comprised of different materials. The spool 120 may bemanufactured using any of various known processes, such as a straightpull mold or other molding process.

The spool 120 includes a first end flange 122, a middle flange 124, asecond end flange 126, and a hub 128. The hub 128 of the spool 120 isgenerally cylindrical in shape and provides a coil retaining surface forthe pull-in coil 112 and the hold-in coil 114. Although a right circularcylinder is shown in the embodiment of FIG. 1, it will be recognizedthat the hub 128 make take on other forms, including cylindrical andnon-cylindrical forms. Furthermore, the term “spool” as used hereinrefers to any appropriate solenoid coil holder, regardless of whetherthe hub is provided as a cylinder or if flanges are included on the endsof the hub.

The hub 128 in the embodiment of FIG. 2 extends from the first endflange 122 to the second end flange 126. The hub 128 defines acylindrical interior passage 130 that extends through the spool 120 fromthe first end flange 122 to the second end flange 126. The cylindricalhub 128 also defines a spool axis 132 that extends through the interiorpassage 130. The spool axis 132 defines a centerline for the spool 120and an axial direction along the spool.

The first end flange 122 provides an end wall for the spool 120 that isconfigured to retain coil windings on the spool. The first end flange122 is generally disc shaped and includes a circular center hole at theinterior passage 130 of the spool. This end wall may be solid with acentral hole for the plunger passage 130, as shown in FIG. 2, or mayinclude a plurality of openings. Moreover, although the flange 122 isshown as a relatively thin circular disc in the embodiment of FIG. 2, itwill be recognized that the end flange 122 may be provided in variousdifferent forms and shapes.

The middle flange 124 also provides a wall that is configured to retaincoil windings on the spool. The middle flange 124 is positioned on thehub 128 between the first end flange 122 and the second end flange 126,but not necessarily centered between the first end flange 122 and thesecond end flange 126. Indeed, in the embodiment of FIG. 2, the middleflange 124 is positioned closer to the second end flange 126 than to thefirst end flange 122. The space between the first end flange 122 and themiddle flange 124 provides a first coil bay 142 on the spool 120 wherethe pull-in coil 112 is wound around the hub 128.

Similar to the first end flange 122, the middle flange 124 in theembodiment of FIG. 2 is also disc shaped. The middle flange 124 isgenerally thicker than the first end flange and includes coil mountingfeatures 134 such as slots 136 along the outer perimeter of the flange124. These slots 136 provide a passage for wire leads on the pull-incoil 112. It will be recognized that additional coil mounting features134 are also possible, and examples of such coil mounting features willbe discussed in further detail below with reference to FIGS. 6-12.Although the center flange is shown in FIG. 2 as having a circularperimeter, it will be recognized that the middle flange 124 may beprovided in various different forms and shapes. For example, althoughthe middle flange 124 is shown as being solid with a single centralopening, the middle flange may also include a plurality of openings.

The second end flange 126 provides another end wall for the spool 120that is configured to retain coil windings on the spool. The spacebetween the second end flange 126 and the middle flange 124 provides asecond coil bay 144 on the spool that is adjacent to the first coil bay142 in the axial direction. The hold-in coil 112 is wound around the hub128 at the second coil bay 144. Similar to the first end flange 122, thesecond end flange 126 is also generally disc shaped and includes acircular center hole at the interior passage 130 of the spool. Thesecond end flange 126 is generally the same thickness as the first endflange 122. Similar to the middle flange 124, includes mounting features134 such as slots 138 along the outer perimeter of the flange 126. Theseslots 138 provide a passage for wire leads on the pull-in coil 112 andthe hold-in coil 114. The second end flange 126 may be solid, as shownin FIG. 2, or may include a plurality of openings. Moreover, althoughthe second end flange 126 is shown as a relatively thin circular disc inthe embodiment of FIG. 2, it will be recognized that the flange 126 maybe provided in various different forms and shapes.

As described above with reference to FIG. 2, the spool 120 of thesolenoid 110 is configured such that the pull-in coil 112 is positionedadjacent to the hold-in coil 114 of the solenoid in the axial direction.As a result of this adjacent coil arrangement, greatly increased fluxleakage can occur around the pull-in coil, as described below withreference to FIGS. 3-5. The increased flux leakage reduces the magneticforce experienced by the plunger as a result of the pull-in coil 112,thus allowing the resistance of the pull-in coil 112 to be low whilestill minimizing the abutment force issues previously described. At thesame time, the adjacent coil arrangement provides for minimal fluxleakage with the hold-in coil 114 when the plunger gap is zero and thecontacts are closed, thus allowing the number of coil turns in thehold-in coil to be low but maximizing its hold-in force.

FIGS. 3-5 are diagrams illustrating lines of magnetic flux through thesolenoid when the pull-in coil 112 and the hold-in coil 114 are invarious energized and non-energized states. In each of FIGS. 3-5, thepull-in coil 112, hold-in coil 114, plunger 116, solenoid case 150 andplunger stop 152 are illustrated as a cross-sectional view of thesolenoid taken radially outward from the solenoid centerline 132. Thesolenoid spool 120 of FIG. 2 is not illustrated in FIGS. 3-5 forclarity, allowing the lines of magnetic flux 170 passing through thesolenoid 110 to be more clearly displayed. However, it will berecognized that the spool 120 is present in the illustrations of FIGS.3-5 with the pull-in coil 112 and hold-in coil 114 wound around thespool, and the plunger 116 inserted in the interior passage 130 of thespool 120.

With particular reference to FIG. 3, the solenoid 110 is housed by thesolenoid case 150. The plunger stop 152 is a generally disc shapedmember that is fixed to the solenoid case 150 and extends radiallyinward from the solenoid case. The plunger stop 152 includes acylindrical protrusion 154 that fits within an end of the interiorpassage 132 of the spool 120 (not shown in FIG. 3). This cylindricalprotrusion 152 provides a stop surface 154 configured to engage theplunger 116 when the plunger is moved in the axial direction by thepull-in coil 112.

The plunger 116 is a solid component with a cylindrical shape. Thecylindrical shape of the plunger 116 is provided with a first largerdiameter portion 160 and a second smaller diameter portion 162. Ashoulder 164 is formed between the larger diameter portion 160 and thesmaller diameter portion 162. The plunger 116 is slideably positionedwithin the solenoid case 150. In particular, the plunger 116 isconfigured to slide in the axial direction along the centerline 132 toclose an air gap 168 (which may also referred to herein as a “plungergap”) between the plunger shoulder 164 and the stop surface 154 of theplunger stop 152. Each of the plunger 116, the solenoid case 150, andthe plunger stop 152 are comprised of a metallic material havingrelatively low magnetic reluctance, such that magnetic flux lines mayeasily pass through the solenoid case and the plunger.

With continued reference to FIG. 3, the pull-in coil 112 of the solenoid110 is positioned within the solenoid case 150 and encircles the largerdiameter portion 160 of the plunger 116. The pull-in coil 112 is removedfrom the plunger stop by a distance d in an axial direction. An axialend of the pull-in coil is aligned with the shoulder 164 of the plunger116 when the plunger is in the leftmost position of FIG. 3. As discussedpreviously, the pull-in coil 112 is comprised of a length of conductorincluding a plurality of windings that wrap around the spool 120 (notshown in FIG. 3). When the pull-in coil 112 is initially energized, theplunger 116 is urged in the axial direction to the right, as indicatedby arrow 166.

The hold-in coil 114 is positioned adjacent to the pull-in coil 112 inthe axial direction within the solenoid case 150. The hold-in coil 114encircles the protrusion 154 of the plunger stop 152 and the associatedstop surface 156. Accordingly, the hold-in coil 114 also encircles thesmaller diameter portion 162 of the plunger that extends through theplunger stop 152. Furthermore, the pull-in coil encircles the air gap168 when the plunger is in the leftmost position of FIG. 3. As discussedpreviously, the hold-in coil 114 is comprised of a length of conductorincluding a plurality of windings that wrap around the spool 120 (notshown in FIG. 3). When the hold-in coil 114 is initially energized, theplunger 116 is urged in the axial direction to the right, as indicatedby arrow 166.

Coil Position Within the Solenoid Results in Leakage

As represented by flux lines 170 in FIGS. 3 and 4, when the pull-in coil112 and the hold-in coil 114 are energized, magnetic flux is createdwithin the solenoid. Leakage flux is any flux that does not contributeto the axial force acting on the plunger 116. The axial force acting topull the plunger 116 toward the plunger stop 152 and close the plungergap 168 is dependent upon the total flux linkage between the pull-incoil 112 and the plunger 116 and between the hold-in coil 114 and theplunger 116. When flux leakage occurs, the flux linkage is reduced andso is the resulting force on the plunger 116.

By placing the pull-in coil 112 away from the plunger gap 168 andplunger stop surface 156, as shown in FIGS. 3 and 4, the flux leakage ofthe pull-in coil 112 is intentionally greatly increased in order toreduce the resulting force on the plunger 116. As shown in FIGS. 3 and4, rather than traverse directly from the plunger 116 to the plungerstop 152, an increased amount of flux by-passes the plunger 116 andcouples directly from one side of the case 150 to the stop 152 or evenback to the case 152 outside wall 151. Examples of this leakage flux areindicated in FIG. 3 by lines 171. The leakage flux 171 effectivelylowers the magnetic force on the plunger 116 for a given amp-turnexcitation of the pull-in coil 112. Since the magnetic force on theplunger 116 is reduced, and because the pinion gear is mechanicallyconnected to the plunger via the pivoting shift lever, the impact andsteady-state abutment force of the pinion gear on the ring gear is alsoreduced. Therefore with the embodiment of FIGS. 1-5, the resistance ofthe pull-in coil 112 can be made low to increase soft start current tothe electric motor 102. Accordingly, the torque of the electric motor102 is increased during soft start, without having excessive abutmentforce between the pinion gear and the ring gear which traditionallyresults from the high amp-turn excitation of the pull-in coil 112.

While coil arrangement in the embodiment of FIGS. 1-5 is configured toincrease the leakage flux for the pull-in coil 112, the arrangement isconfigured to do the opposite for the hold-in coil 114. In particular,the hold-in coil 114 in FIGS. 1-5 is configured to minimize flux leakagewith the plunger 116 in order to maximize the electromagnetic hold-inforce on the plunger 116 for a given number of turns of the hold-in coil114. This is accomplished by centering the hold-in coil 114 at theplunger stop surface 156 interface. In this fashion leakage flux 171 isminimized with the hold-in coil 114, and the electromagnetic force onthe plunger is maximized. Accordingly, by the geometrical layout of thewindings of the pull-in coil 112 and the hold-in coil 114, it ispossible to reshape the force-travel curves of the plunger 116 to valuesmore desirable for a starter with a soft start system.

In addition to the benefits related to flux leakage, the side-by-sidearrangement for the pull-in coil 112 and the hold-in coil 114 can alsohave thermal benefits. In particular, with the conventional coil overcoil winding such as that shown in FIG. 15, the hold-in coil 214 suffersin strength if the abutment time between the pinion gear 206 and thering gear is prolonged. During a prolonged abutment, the pull-in coil212 will rapidly heat and then increase the temperature of the hold-incoil 214. When the temperature of the hold-in coil 214 increases, theelectrical resistance increases and the current decreases. Thisdecreases the resulting hold-in force provided by the hold-in coil andthus the risk of the plunger contacts opening and plunger disengagementis increased. However, with the side-by-side coil arrangement shown inthe starter embodiment of FIGS. 1-5, the thermal influence of thepull-in coil 112 on the hold-in coil 114 during starting is minimal, asthe thermal conductive path resistance is much higher with the two coilsseparated from one another in the axial direction.

Spool with Additional Mounting Features

With reference now to FIGS. 6-7, an alternative embodiment of the spool120 of FIG. 2 is shown. Similar to the spool of FIG. 2, the alternativeembodiment of the spool also generally includes a first end flange 122,a middle flange 124, a second end flange 126, and a hub 128. The hub 128is generally cylindrical about an axial centerline 132, and an interiorpassage 130 extends through the hub from one end of the spool 120 to theother. However, as explained in further detail below, in the embodimentof FIGS. 6-7, the middle flange 124 and the second end flange 126include a number of additional mounting features 134.

FIGS. 6A and 7 show views of the side of the middle flange 124 thatfaces the first coil bay 142. The middle flange 124 includes variousmounting features including a first winding post 172 positioned betweena lead-in slot 174 and a lead-out slot 176. The first winding post 172extends radially outward from the centerline of the spool 120 and isconfigured to engage the wire from the hold-in coil. Sufficient space isprovided around the first winding post 172 to allow the hold-in coil 114to be wrapped around the winding post. Moreover, the first winding post172 is sufficiently long to allowing wire from the hold-in coil 114 tobe wrapped around the first winding post 172 several times. Accordingly,as explained in further detail below, the first winding post 172provides a mounting feature 134 that allows the hold-in coil to besecurely anchored to the spool 120 and also provides a feature forreversing the direction of the turns of the hold-in coil 114 on thespool. A reverse turn post may be advantageous in solenoids for starterswith soft start systems, as described in U.S. patent application Ser.No. 12/767,710, filed Apr. 26, 2010, the content of which isincorporated herein by reference in its entirety.

With continued reference to FIGS. 6A and 7, the lead-in slot 174provides an axial groove in the outer circumference of the middle flange124 which is designed and dimensioned to receive the wire used to formthe pull-in coil 112. Additionally, in the embodiment of FIGS. 6A and 7,the lead-in slot 174 includes an entry ramp 175 for the start lead ofthe pull-in coil 112. This entry ramp 175 extends in a substantiallyradial direction to the hub 128 of the spool 120. The entry ramp 175 isconfigured such that the depth of the slot 174 into the middle flange124 is slightly tapered moving toward the hub 128. Accordingly, thelead-in slot 174 with entry ramp 175 allows the start lead of thepull-in coil 112 to be guided on the spool 120 from the perimeter of themiddle flange 124 toward the hub 128 without consuming space in thefirst coil bay 142 before the start lead reaches the hub 128. Once thestart lead does reach the hub 128, the first layer of turns for thepull-in coil 112 begin. While the lead-in slot 174 has been disclosed asincluding the entry ramp 175, it will be recognized that in at least onealternative embodiment, the lead-in slot extends directly to the hubwithout the entry ramp 175 positioned in the slot 174.

Similar to the lead-in slot 174, the lead-out slot 176 provides anotheraxial groove in the outer circumference of the middle flange 124 whichis designed and dimensioned to receive the wire used to form the pull-incoil 112. However, unlike the lead-in slot 174 in the embodiment ofFIGS. 6A-7, the lead-out slot 176 does not include a ramp portion thatextends in the radial direction to the hub 128 of the spool. Instead,the lead-out slot 174 is simply provided on the perimeter of the middleflange 124 and extends radially approximately the thickness of the wirefor the pull-in coil in order to allow the finish lead of the pull-incoil to cut across the middle flange 124 once the pull-in coil iscompletely wound in the first coil bay 142.

With reference now to FIG. 6B, the opposite face of the middle flange124 is shown. The face of the middle flange 124 shown in FIG. 6B is theface presented to the second coil bay 144 of the spool 120. The firstwinding post 172, the lead-in slot 174, and the lead-out slot 176 areall visible on this side of the middle flange 124. In addition, thisside of the middle flange 124 includes an entry ramp 182 for the startlead of the hold-in coil 114. This entry ramp 182 is similar to theentry ramp 175 for the pull-in coil, extending in a generally radialdirection toward the hub 128 and gradually tapering as the ramp extendstoward the hub 128. Furthermore, the side of the middle flange 124 shownin FIG. 6B includes a second winding post 178 that is only accessible onthis side of the middle flange 124. Accordingly, an indentation 180 isformed in this face of the middle flange 124, and the second windingpost 178 is situated in this indentation 180. As explained in furtherdetail below, this second winding post 178 provides a mounting featurefor the hold-in coil 114 that may be used as an anchor or a reversingturn feature.

With reference now to FIG. 6C the second end flange 126 includesadditional mounting features, including a dual start lead slot 184, afirst finish lead slot 186, and a second finish lead slot 188. The dualstart lead slot 184 is designed and dimensioned to allow the start leadsfor both the pull-in coil 112 and the hold-in coil 114 to pass throughthe perimeter of the second end flange 126. When both start leads arepositioned in the slot 184, the start lead for the hold-in coil 114 ispositioned radially inward from the start lead for the pull-in coil 112.The first finish lead slot 186 is configured to allow the finish leadfor the pull-in coil 112 to pass through the perimeter of the second endflange 126. Similarly, the second finish lead slot 188 is configured toallow the finish lead for the hold-in coil 114 to pass through theperimeter of the second end flange 126.

It will be recognized that the middle flange 124 is thicker in the axialdirection than the two end flanges 122 and 126. This increased thicknessnaturally follows because of the desired separation of the pull-in coil112 and the hold-in coil 114 in the axial direction such that the coilsare properly positioned on the spool 120. However, the increasedthickness also provides increased space for the various coil mountingfeatures 134 included on the middle flange 124. Without this middleflange design, the end flanges 122, 126 would need to be the thicknessof the center flange to provide the same features, and this woulddecrease the available space for the coil bays 142, 144.

The winding of the pull-in coil 112 and the hold-in coil 114 on thespool 120 is now described with reference to FIGS. 8-12 in order toprovide a better understanding of the design of the foregoing mountingfeatures 134 of the spool 120 and arrangement of the coils 112 and 114on the spool.

The process of winding the spool 120 begins with the hold-in coil 114.FIG. 8 shows the hold-in coil 114 being wound in the second coil bay 144of the spool. To begin the winding process, a start lead 190 of thehold-in coil 144 is wrapped around the first winding post 172 in orderto anchor the wire for the hold-in coil to the spool 120. The start lead190 is then channeled down the entry ramp 182 (not shown in FIG. 8) onthe middle flange 124 toward the hub 128. After the start lead 190reaches the hub 128, the spool 120 is rotated in the direction of arrow191, causing a length of wire from a reel (not shown) to be wound aroundthe hub, and create winding turns for the hold-in coil 114. Thesewinding turns are wound in a first turn direction in the second coil bay144 of the spool 120.

As shown in FIG. 9, after a predetermined number of turns in the firstdirection are created in the second coil bay 144, the length of wire forthe hold-in coil is wrapped around the first winding post, and the spool120 is rotated in the opposite direction as indicated by arrow 192.Rotation of the spool in the direction of arrow 192 results in reversewinding turns being created in a second direction in the second coil bay144 of the on the spool 120. Such reverse winding turns may beadvantageous on the hold-in coil in a vehicle starter, as described inU.S. patent application Ser. No. 12/767,710, filed Apr. 26, 2010, thecontent of which is incorporated herein by reference in its entirety.

With reference now to FIG. 10, after the reverse winding turns arecreated, the wire for the hold-in coil is wrapped around the secondwinding post 178 on the middle flange to securely anchor the hold-incoil in the second coil bay 144. The finish lead 194 of the hold-in coilis then directed through the second finish lead slot 188 on the secondend flange 126. The start lead 190 is also directed through the dualstart lead slot 184 on the second end flange 126, and this completes thehold-in coil 114 on the spool 120.

FIG. 11 shows the pull-in coil 112 being wound in the first coil bay 142of the spool 120 after the hold-in coil 114 is wound in the second coilbay 144. To begin winding the pull-in coil, a start lead 196 of thepull-in coil 144 is routed through the dual start lead slot 184 on thesecond end flange 126 and through the lead-in slot 174 on the middleflange 124. The start lead 196 is then directed down the entry ramp 175on the middle flange 124 toward the hub 128. After the start lead 196reaches the hub 128, the spool 120 is rotated in the direction of arrow197, causing a length of wire from a reel (not shown) to be wound aroundthe hub, and create winding turns for the pull-in coil 112 in the firstcoil bay 142 of the spool 120.

With reference now to FIG. 12, after the turns of the pull-in coil 112are completely wound in the first coil bay 142, the finish lead 198 isrouted through the lead out slot 176 on the middle flange 124. Thefinish lead 198 is then directed across the turns of the hold-in coil114 and through the first finish lead slot 186 on the second end flange126. This completes the winding of the pull-in coil 112 on the spool120.

Coil Comprised of Rectangular Wire

FIG. 13 shows a cross-sectional view of the spool 120 along line D-D ofFIG. 12. In this embodiment of the solenoid 110, the pull-in coil 112 iscomprised of rectangular wire 146 (i.e. wire having a substantiallyrectangular cross-section), and the hold-in coil 114 is comprised oftraditional round wire 147. In particular, the rectangular wire 146 usedfor the pull-in coil 112 is square wire in the embodiments of FIGS. 12and 13. The rectangular wire 146 is jacketed with a layer of insulationon the outer perimeter. The wire 146 also includes slightly radiusedcorners 148 that are provided for manufacturing concerns and to avoidany sharp edges on the wire which might cut into the insulation layer onneighboring wires. As explained below, the rectangular wire 146 isadvantageous for use in the pull-in coil 112, as it provides anincreased stacking factor for the coil while also providing thermalbenefits for the coil.

The stacking factor for a coil is the ratio of the total volume consumedby conductors only (i.e., not including air voids between conductors) tothe total volume consumed by the complete coil (i.e., including allconductors and air gaps between conductors). Traditional round wire hasan effective stacking factor of about 78%. In contrast, the square wiredisclosed herein has an effective stacking factor of 90% or more. Inparticular, the square wire 146 used in the embodiment of FIGS. 12 and13 has a stacking factor of 92%. As a result, when comparing square wireand round wire, square wire will require less space to provide the sameelectromagnetic force (i.e., less space to provide the same amp-turns).This space savings is particularly useful for vehicle starters where thestarter is often situated in a crowded engine compartment.

Another benefit of the rectangular wire 146 of FIGS. 12 and 13 is thatit provides a better thermal conduction path than round wire fortransporting the ohmic heat of the coil 112 to the edges of the coil,where the heat may be removed by conduction or convection. With a roundwire coil, there is only point contact between adjacent windings, as theconductors layers are wound on top of each other (i.e., two adjacentcircles will only touch in a single point). In contrast, as shown inFIG. 13, with square wire 146 the interface between conductors onadjacent windings is much larger since there is contact between adjacentconductors along the entire flat portion of the sides of the conductors.Therefore, the heat being transmitted from coil wire to coil wire istransported via the copper wire rather than the air between the wires,and this copper-to-copper conduction provides a significant thermaladvantage. For example, the improved conduction reduces the deltatemperature difference between the outside edges of the coil and thetypical center hot spot of the coil.

With reference now to FIG. 14, yet another alternative embodiment of thesolenoid spool 120 and coils 112, 114 is shown. In this embodiment, thepull-in coil 112 is comprised of rectangular wire 146, and the hold-incoil 114 is also comprised of rectangular wire 149. The rectangular wire146 of the pull-in coil 112 is essentially the same as the rectangularwire 149 of the hold-in coil, but the width of the pull-in coil wire 146is greater than the width of the hold-in coil wire 149. Accordingly, thehold-in coil wire is square wire with radiused corners. Additionally,the rectangular wire 149 is jacketed with a layer of insulation on theouter perimeter. The rectangular wire 149 of the hold-in coil 114 alsoprovides similar advantages to those described above for the pull-incoil 112. For example, the rectangular wire 149 provides an increasedstacking factor for the hold-in coil 114 while also providing thermalbenefits for the coil.

Variable Reluctance Plunger

In the above-described embodiments, the plunger 116 has been depicted ashaving a larger diameter portion 160 and a smaller diameter portion 162,with each portion having a constant cross-sectional profile (i.e. aconstant diameter). However, in at least one alternative embodimentshown in FIG. 15, the diameter of the larger diameter portion 160 of theplunger 116 is varied such that a radial gap 70 between a guide plate 72and the plunger 116 varies as the plunger slides in the axial direction.As a result, the reluctance of the magnetic circuit and the relatedmagnetic force on the plunger can be tailored during the pull-in andhold-in regions of operation, as explained in more detail below.

As shown in FIG. 15, the larger diameter portion 160 of the plunger 116includes a substantially cylindrical outer surface 80 with acircumferential notch 82 formed therein. The circumferential notch 82includes a central cylindrical section 84 with tapered edges 86. Thediameter of the central cylindrical section 84 of the circumferentialnotch 82 is smaller than the diameter of the remainder of thesubstantially cylindrical outer surface 80 (the diameter being definedin a radial direction, perpendicular to arrow 166). As shown in FIG. 15,the substantially cylindrical outer surface 80 is substantially uniformand extends in the axial direction 166 on both sides of thecircumferential notch 82 in the axial direction 166. In the exemplaryembodiment of FIG. 15, the substantially cylindrical outer surface 80has a constant first diameter along a first portion that extends fromthe shoulder 164 to the circumferential notch 82. At the circumferentialnotch 82 the diameter of the substantially cylindrical outer surface 80changes to a smaller second diameter. Then on the opposite side of thecircumferential notch 82 (i.e., the side of the substantiallycylindrical surface 80 on the opposite side of the circumferential notch82 from the side adjacent to the shoulder 156), the substantiallycylindrical outer surface 80 includes a second portion that is alsodefined by the first diameter. This second portion continues to extendaway from the circumferential notch 82 in the axial direction at somelength.

As shown in FIG. 15, the circumferential notch 82 on the substantiallycylindrical outer surface 80 of the plunger 116 is positioned oppositethe guide plate 72. The guide plate 72 is a generally disc-shapedstructure that extends radially outward from the plunger 116. In theembodiment of FIG. 15, the guide plate 72 is provided as part of thesolenoid case 150 and is integral with the outside wall 151 of the case150. The guide plate 72 is positioned adjacent to the end of the spool120 that is furthest removed from the plunger stop 152. Accordingly, thespool 120 is bordered within the solenoid case 150 by the guide plate 72and the plunger stop 152.

In the embodiment of FIG. 15, a thin core tube 76 is positioned alongthe inner diameter of the spool 120 between the plunger 116 and thespool 120. The core tube 76 is fixed in place relative to the spool 120,and the plunger 116 is configured to slide within the core tube 76. Thecore tube 76 extends in the axial direction the full length of the spooland past the guide plate 72. A plunger return spring 78 is positioned onthe opposite side of the guide plate 72 from the spool 120 and engagesboth the guide plate 72 and the core tube 76. The core tube 76 may becomprised of any of various materials such as, for example, brass ornon-magnetic stainless steel.

The extension of the core tube 76 beyond the guide plate 72 of thesolenoid case 150 serves at least two purposes. First, the extended coretube 76 provides a continuous sliding surface for the plunger 116. Thecontinuous surface provided by the core tube 76 prevents the edges 86 ofthe notch 82 from catching on the guide plate 72 during plunger movementwhen the solenoid coils are activated and deactivated. Second, theextended core tube 76 prevents the return spring 78 (which is locatedcoaxially with the plunger 116 and rests against the outside of theguide plate 72) from being lodged into the notch 82 in the plunger 116during activation of the coil 112. If this were to occur, the plunger116 would jam against the guide plate 72 of the case 150.

The plunger 116 shown in FIG. 15 provides a design where the reluctanceof the magnetic circuit and the related magnetic force on the plunger116 is variable during the pull-in and hold-in operations of thesolenoid 110. FIG. 16 provides a graph illustrating the difference inplunger axial force between a standard plunger (i.e., a plunger with nocircumferential notch) and the variable reluctance plunger 116 of FIG.15 as the axial plunger gap 168 is closed. In this graph, the top solidline curve 60 represents the axial force of the standard plunger with aconstant diameter, and the dotted line curve 62 represents the axialforce of the variable reluctance plunger 116 with a circumferentialnotch 82, such as that of the embodiment of FIG. 15. The lower solidline 64 in FIG. 16 represents the axial force of the return spring.Between positions A and C, the lower solid line 64 represents the axialforce of the return spring, and between positions C and D, the lowersolid line 64 represents the axial force of both the return spring and acontact over-travel spring.

Four different positions of the plunger are specifically noted in FIG.16. These four positions include the following: position A at the startof the pull-in operation where the axial plunger gap 168 is at a maximumand the pull-in coil 112 and the hold-in coil 114 are initiallyenergized; position B in the middle of the pull-in operation when boththe pull-in coil 112 and the hold-in coil are energized; position C atthe end of the pull-in operation and start of the hold-in operationwhere the plunger 116 has moved to a position where the plunger contactsare closed and the pull-in coil 112 is shorted; and position D duringthe hold-in operation where axial plunger gap 168 is completely closedan only the hold-in coil 114 is energized.

With continued reference to FIG. 16, the net work on the plunger duringthe pull-in operation is the integral of the net force (magnetic forceminus spring force) acting on the plunger over the activation length.This is represented by the area under each of curves 60 and 62 in FIG.16. This energy is dissipated during impact between the starter drivepinion and ring gear during an abutment.

FIGS. 17-20 show movement of the plunger 116 with circumferential notch82 at each of the four positions A, B, C and D of FIG. 16. FIG. 17 showsthe plunger in position A where the axial plunger gap 168 is at amaximum distance and the pull-in coil 112 and the hold-in coil 114 areinitially energized. At this position A, the notch 82 of the plunger 116is positioned opposite the guide plate 72. In particular, the guideplate 72 is positioned directly across from the front tapered edge 86 ofthe notch 82 in position A. With the plunger 116 in this position, theradial gap 70 between the plunger 116 and the guide plate 72 ismaximized. Because of this large radial gap 70, the resulting magneticreluctance of the solenoid magnetic circuit is increased. As a resultthe axial force on the plunger 116 is significantly reduced, as noted bycomparing curves 60 and 62 in FIG. 16.

FIG. 18 shows the plunger in position B where the axial plunger gap 168is closing as the plunger 116 moves toward the plunger stop 152, withboth the hold-in coil 112 and the pull-in coil remaining energized. Atthis position B, the notch 82 of the plunger 116 remains opposite theguide plate 72. In particular, the guide plate 72 is positioned directlyacross from the rear tapered edge of the notch 82 in position B. Withthe plunger 116 in this position, the radial gap 70 between the plunger116 and the guide plate 72 remains maximized. Because of this largeradial gap 70, the resulting magnetic reluctance of the solenoidmagnetic circuit remains increased, as the axial force on the plunger116 is still significantly reduced over that of the standard plunger.

FIG. 19 shows the plunger 116 in position C where the axial plunger gap168 is nearly closed and the plunger 116 has moved the plunger contacts117 (see FIG. 1) to a closed position. With the plunger contacts 117closed, the pull-in coil 112 is shorted, as described previously withrespect to FIG. 1. Additionally, the plunger notch 82 at position C isno longer directly opposite the guide plate 72, but has moved past theguide plate 72 in the axial direction. With the plunger 116 in thisposition, the radial gap 70 between the plunger 116 and the guide plate72 is minimized. Because of this small radial gap 70, the resultingmagnetic reluctance of the solenoid magnetic circuit is decreased,allowing the hold-in coil 114 to produce the maximum axial force on theplunger 116.

FIG. 20 shows the plunger in position D where the hold-in coil 114remains energized and the pull-in coil 112 remains shorted. At thisposition D, the axial plunger gap 168 is completely closed, as thehold-in coil 114 has moved the plunger 116 into engagement with theplunger stop 152, overcoming both the force of the plunger return springand the contact over-travel spring (not shown). Moreover, the plungernotch 82 at position D is even further removed from the guide plate 72in the axial direction such that the radial gap 70 between the plunger116 and the guide plate 72 remains minimized. Accordingly, with theplunger in this position D, the hold-in circuit 114 is allowed toproduce the maximum axial force on the plunger.

As described above with reference to FIGS. 15-20, through the use of avariable reluctance plunger, the net work during the pull-in region canbe reduced while still accomplishing all of the functional needs of thesolenoid. The variable cross-section plunger provides for a variablereluctance magnetic circuit that results in increased magneticreluctance during the pull-in operation (i.e., compare curves 60 and 62between points A and C in FIG. 16). Accordingly, the axial force on theplunger is reduced during the pull-in operation, thus reducing theenergy dissipated during an impact between the starter drive pinion andthe ring gear during an abutment. At the same time, the variablecross-section plunger provides for a variable reluctance magneticcircuit that provides for increased (i.e., standard) magnetic reluctanceduring the hold-in operation (i.e., compare curves 60 and 62 betweenpoints C and D in FIG. 16). This allows the axial force on the plungerto remain at an increased level during the hold-in operation.

While only one embodiment is shown in FIGS. 15-20, it will be recognizedthat there are numerous alternative embodiments for the plunger,including many possible alternative geometric configurations, whichwould exhibit the desired magnetic behavior and may be tailored for agiven application and need. Moreover, while the variable reluctanceplunger 116 has been described in FIGS. 15-20 as used in associationwith a hold-in coil and a pull-in coil that are axially adjacent, in aother embodiments the variable reluctance plunger may be used inassociation with a more traditional winding arrangement where thehold-in coil is radially adjacent to the pull-in coil (i.e., hold-incoil wound around the pull-in coil) such as the coil arrangement shownin FIG. 23, or vice-versa (i.e., where the pull-in coil is wound aroundthe hold-in coil).

Variable Reluctance Plunger Sleeve

At least one alternative embodiment for the variable reluctance plungerof FIG. 15 is shown in FIGS. 21 and 22. As shown in FIG. 21, acylindrical sleeve 92 is positioned over the circumferential notch 82 onthe larger diameter portion 160 of the plunger 116. The sleeve 92 isslightly larger in diameter than the plunger 116 and may include taperededges 94. Accordingly, the sleeve 92 may serve as the surface contactbetween the plunger 116 and the core tube at this end of the solenoid.However, in other embodiments, the sleeve 92 may be substantially thesame diameter as the remainder of the large diameter portion 160 or evenslightly smaller in diameter.

In at least one embodiment, the sleeve 92 is formed as an over moldedplastic sleeve on the plunger 116. In this embodiment, the sleeve 92 iscontinuous and substantially smooth over its entire outer surface 96, asshown in FIG. 21A. In another embodiment, the sleeve 92 is a separatelymolded plastic sleeve that is inserted into place over the plunger 116.In this embodiment, the sleeve 92 may include an axial slot 98 to allowthe sleeve 92 to increase in diameter when it is inserted over theplunger 116, and then resiliently snap back when the sleeve 92 reachesthe circumferential notch 82, thus allowing the sleeve 92 to be securedon the plunger 116.

The sleeve may be formed from any of numerous different materials,including, for example, stainless steel or brass. In at least oneembodiment, the sleeve is comprised of polytetrafluoroethylene (PTFE),which is marketed commercially under the name TEFLON®. PTFE provides alow coefficient of friction for the sleeve during plunger movement.

As shown in FIG. 22, by covering the circumferential notch 82 on theplunger 116 with a sleeve 92 (shown in dotted lines in FIG. 22 in orderto show its position over the notch 82), the core tube 76 does not needto extend past the guide plate 72 of the solenoid case 150. This is incontrast to the embodiment of FIG. 15 where the core tube 76 extendscompletely past the guide plate 72. In FIG. 15, the existence of theelongated core tube requires the radial gap between the plunger 116 andguide plate 72 to be increased in order to accommodate the core tube.This increases the reluctance of the magnetic circuit. This increase inmagnetic reluctance does not degrade the design of the solenoid 110during the pull-in operation since, as described previously, it may beadvantageous to increase magnetic reluctance during the pull-inoperation. However, this increase in magnetic reluctance may presetdisadvantages during the hold-in operation since, as also describedpreviously, it is advantageous to decrease magnetic reluctance duringthe hold-in operation in order maximize the flux and therefore axialforce of the plunger. However, in the embodiment of FIG. 22, thisadditional radial gap distance required to accommodate the extended coretube 76 is eliminated. By reducing the radial gap distance, thereluctance of the magnetic circuit is also reduced. With reducedmagnetic reluctance, the resulting axial force on the plunger isincreased during the hold-in operation. During the pull-in operation,the circumferential notch 82 acts to provide the desired increasedmagnetic reluctance.

In addition to the foregoing advantages, the sleeve 82 also eliminates arequirement for the full plunger diameter to overlap a portion of theguide plate for vibration reasons and creasing of the core tube. Inparticular, the plunger 116 is supported by the interface between thesleeve 92 and the guide plate 72. Moreover, this allows thecircumferential notch in the plunger 116 to be extended and furtherreduces the excessive pull-in magnetic force, particularly at the ‘atrest’ position.

In addition to the issues addressed by the sleeve 92 itself, a PTFEsleeve is particularly helpful with other issues. For example, when theplunger 116 is at rest a portion of the plunger should be at fulldiameter within the region of the guide plate 72 so that the vibrationforce is transmitted from the plunger 116 to the guide plate 72. If not,the vibrational force of the plunger 116 against the unsupported,cantilevered core tube will cause a crease to form in the thin tube.Over time this crease and fretting will require a high mechanical forceto overcome it during activation and deactivation and may become highenough to cause solenoid functional failure. A PTFE/stainless steelinterface is generally superior to a carbon steel/stainless steelinterface for fret resistance. During vehicle or equipment use, theplunger is at the ‘at rest’ condition. As a result of vibration, theplunger/core tube interface is constantly under small amplitudescrubbing action where the two parts move relative to each other. Thiscan lead to fretting damage. However, when one of the surfaces is PTFE,such as the PTFE sleeve 92 described above, the abrasiveness of thesteel-on-steel is eliminated and this problem minimized. Additionally,the lubricity of PTFE helps during engagement and disengagement of thesolenoid. In particular, PTFE is more likely to provide a trouble freesliding surface that is not damaged by corrosion on the surface whichcan sometimes be problematic a conventional metal-on-metal arrangement.

The foregoing detailed description of one or more embodiments of thestarter motor solenoid with variable reluctance plunger been presentedherein by way of example only and not limitation. It will be recognizedthat there are advantages to certain individual features and functionsdescribed herein that may be obtained without incorporating otherfeatures and functions described herein. Moreover, it will be recognizedthat various alternatives, modifications, variations, or improvements ofthe above-disclosed embodiments and other features and functions, oralternatives thereof, may be desirably combined into many otherdifferent embodiments, systems or applications. Presently unforeseen orunanticipated alternatives, modifications, variations, or improvementstherein may be subsequently made by those skilled in the art which arealso intended to be encompassed by the appended claims. Therefore, thespirit and scope of any appended claims should not be limited to thedescription of the embodiments contained herein.

What is claimed is:
 1. A solenoid for a vehicle starter, the solenoidcomprising: a first coil and a second coil with a passage extendingthrough the first coil and the second coil in an axial direction,wherein the first coil is a pull-in coil and the second coil is ahold-in coil; and a plunger configured to move in the axial directionwithin the passage, the plunger including a cylindrical outer surfacewith a substantially uniform diameter and a circumferential notch, thecylindrical outer surface including a first portion with a firstdiameter on one side of the circumferential notch, and a second portionwith the first diameter on an opposite side of the circumferentialnotch, the circumferential notch including a portion with a seconddiameter that is less than the first diameter.
 2. The solenoid of claim1, the plunger configured to move in an axial direction between a firstposition where the first portion of the plunger is removed from aplunger stop and a second position where the first portion of theplunger engages the plunger stop.
 3. The solenoid of claim 2 furthercomprising a radially extending plate member separated from the plungerby a radial distance, wherein the radial distance varies when theplunger moves from the first position to the second position as a resultof the circumferential notch on the outer surface of the plunger movingin relation to the plate member.
 4. The solenoid of claim 1, the firstportion having an axial length that is greater than an axial length ofthe at least one of the first coil and the second coil.
 5. The solenoidof claim 1, the first portion having an axial length that issubstantially the same as an axial length of least one of the first coiland the second coil.
 6. The solenoid of claim 1 wherein the portion withthe second diameter is a cylindrical portion.
 7. The solenoid of claim 1wherein the first coil is axially adjacent to the second coil.
 8. Thesolenoid of claim 1 wherein the first coil and the second coil are woundon a spool, and the passage extends through the spool.
 9. The solenoidof claim 8 further comprising a plate member positioned adjacent to anend of the spool, the plate member separated from the plunger by aradial distance, wherein the radial distance varies when the plungermoves as a result of the notch moving in relation to the plate member.10. The solenoid of claim 8 further comprising a tube positioned withinthe spool with the plunger positioned within the tube and configured tomove in the axial direction relative to the tube, wherein the tubeextends past a plate member at the one end of the spool.
 11. Thesolenoid of claim 10 further comprising a sleeve member coupled to theplunger and covering the circumferential notch in the plunger.
 12. Thesolenoid of claim 11 wherein the sleeve member is molded over thecircumferential notch.
 13. The solenoid of claim 12 wherein the sleevemember includes a split groove configured to allow the sleeve member toexpand when the sleeve member is coupled to the plunger.
 14. Thesolenoid of claim 13 wherein the sleeve member has a greater diameterthan the first diameter.
 15. The solenoid of claim 14 wherein the sleevemember is comprised of a polymer material.
 16. The solenoid of claim 1,wherein the plunger is void of a permanent magnet.
 17. A solenoid for avehicle starter, the solenoid comprising: a spool; a pull-in coil and ahold-in coil wound on the spool with a passage extending through thespool, the pull-in coil and the hold-in coil; and a substantially solidplunger positioned within the passage and configured to slide within thepassage in an axial direction between a first position and a secondposition, the plunger including a cylindrical outer surface with acircumferential notch formed in the cylindrical outer surface between afirst end and a second end of the cylindrical outer surface; and aradial wall separated from the plunger by a radial distance, wherein theradial distance varies when the plunger moves from the first position tothe second position.
 18. The solenoid of claim 17, the cylindrical outersurface of the plunger including a first portion with a first diameteron one side of the circumferential notch, and a second portion with thefirst diameter on an opposite side of the circumferential notch, thecircumferential notch including a portion with a second diameter that isless than the first diameter.
 19. The solenoid of claim 17, the firstportion having an axial length that is greater than an axial length ofthe at least one coil.
 20. A method of operating a solenoid for avehicle starter, the solenoid including a pull-in coil and a hold-incoil wound on a spool, the method comprising: energizing the pull-incoil of the solenoid; moving a substantially solid plunger in an axialdirection as a result of energization of the pull-in coil, thesubstantially solid plunger including a cylindrical outer surface with asubstantially uniform diameter and a circumferential notch formed in thecylindrical outer surface between a first end and a second end of thecylindrical outer surface; and varying magnetic reluctance between theplunger and a solenoid wall as a result of the circumferential notchmoving relative to the solenoid wall.